| dc.description.abstract | The Chegualin fault (CGLF) has been acknowledged as an active fault by the Central Geological Survey since 2021. The fault trace of CGLF mainly appeared in the Gutingkeng formation, which is mainly composed of thickly bedded mudstone. Onsite geodetic measurements suggested that the CGFL has continuous creeping movement. The movement caused damage to the infrastructures straddling the fault trace. But evidence of the fault trace like fault scarps are rarely preserved due to the rapid change of the topography caused by surface erosion in the mudstone region. Through field investigation in the areas which show high surface displacement velocity gradients revealed by geodetic monitoring. Previous research found out that the occurrence of the mudstone in the area was different from others and concluded that the occurrence difference was caused by the faulting of CGLF. Therefore, this research is based on the finding derived from the mesoscale observations by Chen (2021) that black bands with a thickness ranging from several mm to 1 or 2 cm , which is a product of strain localization, appear in argillaceous Chegualin fault rocks. Wall rock and fault rock samples were collected from the same fault outcrop and rock core as Chen (2021). Microscopic observations, mineral assemblage analysis and synchrotron XRD on these samples were performed to obtain understanding of the deformation mechanism of the CGLF.
The microscopic observations show that S-C fabric occur in fault rocks and mineral grains are fractured, deformed and forming preferred orientation in the black bands. Within the black bands, clay minerals alignment along S, C and C’ shear surface and the grain size reduction of quartz are observed. These evidence suggests that the black bands are mainly formed by frictional sliding accompanied by cataclasis and serve as deformation bands within the rocks. The density of black bands within the fault rocks can indicate how severe they were deformed.
The whole rock mineral assemblage analysis indicates that the content of clay minerals increase and the crystallinity of illite decreases with the increase of deformation intensity of the rock. The clay mineral assemblage analysis indicates that the fluid-rock interaction occurred within the fault zone. Comparing the EI (Esquevin-indices) of illite in between wall rocks and black fault rocks, we inferred that the smectite-illite transition did not occur within the fault zone. Therefore the decrease of illite crystallinity may result from the generation of defects in the lattices of illite during grain size reduction.
The synchrotron XRD analysis indicates that black bands contain amorphous materials. Based on the result of the aforementioned observations and the previous research about the formation of amorphous materials, we interpreted that the amorphous materials in the black bands were formed through comminution of clasts during the faulting. The fluid-rock interaction occurs since the dehydration of clay minerals during comminution. The grinded minerals rich in mobile elements are dissolved and consumed which let the clay minerals preserved and increase relatively, also the fluid can react with amorphous materials forming smectite.
The forming of the S-C fabric and the increasing content of the clay minerals will weaken the rock strength by reducing the frictional coefficient are documented by experiment test. Therefore we inferred that the deformation caused by the faulting is accommodated by the fault zone of CGLF but mainly localized in the position where black fractured mudstone distributed, forming distributed deformation. The presence of amorphous materials in the fault zone suggests the ongoing amorphization caused by the recent faulting of the CGLF, implicated that the forming of amorphous materials may be related to the creeping movement of the CGLF.
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